Bearings using ptfe as the low friction self lubricating sliding surface have been in regular use in the aerospace industry for 25 years. When such bearings are used in airframes and flying controls, a special construction of the ptfe is required, in order to overcome the inherent cold flow tendencies of the bulk polymer at the high stress levels specified by aerospace designers. For this purpose thin films of woven ptfe bearing surfaces are frequently reinforced with sintered bronze and textile yarns such as glass, graphite fibres, or high strength organic yarns of relatively high melting point (i.e. higher than 250.degree. C.). Woven structures are usually suffused with resin systems such as phenol formaldehyde, epoxies or cyanoacrylates to bind the sliding surface into a dense structure. This resin system can also double as an adhesive system for securing the ptfe layer to a substrate so as to produce a mechanical component for easy installation of the complete bearing into the aircraft structure. Spherical plain self aligning bearings, or cylindrical journal bearings with or without flanges often incorporate ptfe in this form, using hardened steel counterfaces. International standards are widely employed to rationalize the size ranges commercially available off-the-shelf in both inch and metric sizes but planar and more complex shapes are also available as nonstandard items.
This type of bearing is now widely employed and known to be extremely successful in many military and civil fixed wing and variable geometry aircraft where its high load carrying capacity, freedom from maintenance and resistance to fretting damage are important. Its use has also extended to ground based application in fighting vehicles, public and commercial transports, oil and gas pipelines, off-road equipment, marine applications and racing cars.
However, in applications where low stress, high frequency, small amplitude motions occur, as in helicopter rotating assemblies and vehicle suspension systems, the ptfe bearing has so far exhibited a lack of endurance, having a useful life of the order of 5-10 times less than predicted. For example, the track rods and pitch links operating the pitch of helicopter main and tail rotor blades at each revolution of their respective rotors, typically achieve lives of 500-1000 flying hours for main rotors, and 300-600 hours for tail rotors in general terms, after which the ptfe bearings need to be replaced because of unacceptable backlash. In military uses, where 300 hours per annum peacetime operation are averaged, these lives may be acceptable, but such performances are unacceptable to the civil helicopter operator where 1000 hours per annum is the norm. In the latter case, guarantees of 2000 hours plus between replacements and overhaul are required, a bearing performance requirement which has consistently eluded the self-lubricating ptfe bearing manufacturers for the past 10 years or more.
The ptfe liner of a textile type bearing comprises high molecular weight ptfe in long chains making up the individual filaments in each bundle of ptfe multi-filament yarns. The woven textile produced from these yarns is further suffused with resin and possibly other textile fibres and/or fillers such that from 30% to almost 100% of the sliding surface of the resulting liner may be constituted by the ptfe, depending on the particular construction employed, the remainder of the contact surface being constituted by the resin and fillers. The ratio of ptfe to resin at the sliding surface controls the wear resistance of the liner with maximum wear resistance at high stresses and slow oscillations being obtained with a low ratio of ptfe to resin and other constituents.
In the early stages of the life of the bearing, the high molecular weight ptfe undergoes chain scission by the high mechanical forces and thermal degradation at the sliding interface to produce a low molecular weight transfer film or third body, in tribological terms, at the sliding interface. It is this third body which acts as a "grease" in shear which produces the characteristic properties of ptfe sliding bearings and thus its initial formation and its subsequent maintenance are of vital importance to the effectiveness and long life of the bearing. It was recognized 20 years ago that the counterface must be smooth to prevent ploughing and disruption of the third body film so as to preserve a stable system for as long as possible and to prevent ejection of the third body from the ends of the bearings if a low wear rate was to be achieved. Thus hard steel corrosion resistant counterfaces were adopted capable of being finished to approximately 4 micro inches (0.10 microns) centerline average (CLA) with hardness values of around 700 VPN to enable the required surface finish to be obtained more readily.
At high stress levels of the order of 25,000-34,000 lbf/sq. inch (172-234 MPa) at slow oscillating speeds and steady loads such bearings demonstrate excellent wear resistance in accordance with the American and European International Technical standards currently in force, but, as mentioned previously, such bearings are much less satisfactory at low stress levels.
The present invention is based on the surprising discovery, by simulation of the helicopter operating conditions on a test rig, that the wear mechanism at high frequency, light reversing loads with small amplitude oscillations is due initially to roughening and wear of the hard steel counterfaces, which in turn induces a much higher wear rate of the ptfe liner system. Further, the present invention stems from the realization that under the lightly loaded reversing applications typical of helicopter flying controls and transport vehicle suspension systems, the vital third body fails to form rapidly because there is a general lack of adequate thermal and mechanical forces to promote degradation. As a consequence the resin and fillers in the liner severely abrade the surface of the hardened steel ball, despite their relative softness, causing severe damage which in turn ploughs into the ptfe liner causing an excessive and unexpected rate of wear.